The present invention relates to a method for the preparation of a silicone resin. More specifically, the present invention relates to a method for preparing a silicone resin composed of monofunctional siloxane units (M units) and tetrafunctional siloxane units (Q units).
Silicone resins composed of monofunctional (M) and tetrafunctional (Q) siloxane units are used as starting materials for pressure-sensitive adhesives, as reinforcing components for silicone rubber compositions, and as starting materials for film-forming materials such as silicone varnishes, etc. Methods proposed for the preparation of such silicone resins include the following: (a) cohydrolysis by the addition of water to a mixture of alkyl silicate and hydrolyzable trialkylsilane (U.S. Pat. No. 2,857,356), and (b) dripping alkyl silicate into a mixed system of organosilicon compound(s) (selected from disiloxanes and hydrolyzable triorganosilanes) in aqueous hydrochloric acid solution (Japanese Patent Publication Number Hei 3-60851 [60,851/1991]).
To be useful as a film-forming material, this type of silicone resin should have a high-molecular-weight because the development of excellent film-forming properties depends on the molecular weight of the silicone resin. However, when high-molecular-weight silicone resin is prepared by the methods taught in U.S. Pat. No. 2,857,356 and Japanese Patent Publication Number Hei 3-60851 (addition of fewer moles of the hydrolyzable trialkylsilane or disiloxane precursor for the monofunctional siloxane (M) unit than moles of the alkyl silicate precursor for the tetrafunctional siloxane (Q) unit), either the product undergoes gelation or the product has a very poor storage stability when stored in solution form.
The present invention takes as its object the introduction of a method for the preparation of high-molecular-weight silicone resin that has all excellent storage stability in solution form.